Astronomers at NASA have seen the moment that a star was torn apart around a black hole, before its debris crashed back into its own tail in a spectacular "death spiral".
The event is called ASASSN-14li (All Sky Automated Survey for SuperNovae) and was first observed in 2014, having occurred 290 million years ago. A Sun-like star is thought to have strayed too close to a black hole 3 million times more massive than our Sun, where it was then torn apart – known as a tidal disruption event (TDE).
During a TDE, a star’s dust and gas are ripped away and end up swirling around the black hole in a spinning accretion disk. This can become compressed and superheated, circling the black hole’s event horizon, the region beyond which nothing can escape the black hole’s pull, not even light.
While we’ve seen X-ray emissions from these events before, astronomers had been puzzled by flashes of optical and ultraviolet light. But in this latest study, published in The Astrophysical Journal Letters, the researchers used telescopes in Hawaii and Chile – and NASA’s Swift space observatory – to study the ASASSN-14li event.
They found that some debris from the star was overshooting the black hole, causing it to curve back around. It then impacted the incoming debris from the star, causing the bright flashes as the two streams collided.
"We discovered brightness changes in X-rays that occurred about a month after similar changes were observed in visible and UV light," said Dheeraj Pasham, an astrophysicist at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, and lead researcher of the study, in a statement. "We think this means the optical and UV emission arose far from the black hole, where elliptical streams of orbiting matter crashed into each other."
"Returning clumps of debris strike the incoming stream, which results in shock waves that emit visible and ultraviolet light," added NASA Goddard's Bradley Cenko, the acting Swift principal investigator and a member of the science team, in the statement.
It’s a pretty interesting occurrence, and shows us we’ve still got much to learn about how stars interact with black holes.